Previously, attempts have been made at forming mushroom shaped gates of field effect transistors using refractory metals. Mushroom shaped gates typically have a large cross-sectional area to reduce resistance, while the portion of the gate contacting the substrate remains small to reduce capacitance. As a result, transistors with mushroom shaped gates have desirable high-frequency and low-noise characteristics. Typically, mushroom shaped gates are fabricated by forming a mold in several electron sensitive resist layers, wherein metal for the gate could be deposited. However, these resist layers would generally deform if a refractory metal was deposited because of the high temperatures needed to deposit refractory metals, rendering the attempt unsuccessful. As a result, metals such as aluminum and copper are commonly used in the industry because they can be evaporated at a temperature sufficiently low, so that the resist is unaffected. One disadvantage of not using a refractory metal is that high temperature processing cannot be performed after forming the gate. Typical non-refractory gate metals include gold, aluminum, and copper. After the gate has been formed using one of these metals, the metal may diffuse into the substrate if exposed to high temperature processes. These high temperatures are not necessarily the melting point of the metal, but merely high enough to allow the metal to diffuse into the substrate. In addition, gates with these metals have a tendency to diffuse into the substrate over extended periods of time, even without the presence of high temperatures. However, by depositing a refractory metal between the gate and the substrate, the diffusion problem can be avoided.
Prior attempts at incorporating a refractory metal have been attempted, but resulted in complicated techniques. U.S. Pat. No. 5,739,557 O'Niel, LL, et al., “Refractory Gate Heterostructure Field Effect Transistor,” teaches the fabrication of a HFET using a refractory metal gate and a refractory ohmic contact. However, the fabrication scheme disclosed does not allow a metal lift-off step to remove the resist after the gate is formed. This significantly complicates the process. This fabrication scheme requires the use of dielectric deposition and etching techniques which add complexity to device processing and limit its use to materials which are compatible with these techniques. A similar non-lift-off refractory gate fabrication scheme is discussed in T. Suemitsu et al. in Jpn. J. Appl. Phys. Vol. 37 (1998) Pt. 1 No. 3B pp. 1365–1372. Suemitsu describes the fabrication of a refractory metal sub 0.25 micron gate using multiple dielectric layers with multiple reactive ion etching steps and sputtered metal films in a non-lift-off process. As with O'Niel, LL, et al., this technique requires the use of dielectric deposition and etching techniques which add complexity to the process.
As a result, there is a need for a simple process in which a structure is fabricated, wherein a gate comprising a refractory metal can be deposited, followed by removal of resists and extraneous metals using a simple lift-off process.